New compensation system improves machine tool accuracy
13 December 2012
The University of Huddersfield has developed a new version of its volumetric error compensation system for large machine tools and co-ordinate measuring machines.
The work has been carried out in close collaboration with Asquith Butler, the only remaining UK producer of large capacity, multi-axis machining centres whose factory and technical centre is in Brighouse, West Yorkshire.
The Centre for Precision Technologies at the University of Huddersfield has been working in the field of volumetric error compensation for two decades and has gained considerable experience in improving machine tool accuracy. The original error compensation system was successfully installed at a number of aerospace companies, either directly into the control of the machining centre or as a stand-alone unit.
It is noteworthy that the university is the lead organisation of the EPSRC Centre, which includes the National Physical Laboratory and the Science and Technology Facilities Council. Asquith Butler is one of 11 industry partners, alongside such firms as Rolls-Royce, Renishaw and Carl Zeiss.
The new, enhanced Machine Tool Compensation (MTC) system can be integrated into most machining centres, both new and those already in operation, irrespective of controller type. Other prismatic machines including co-ordinate measuring machines may be similarly equipped. The system is capable of improving volumetric accuracy by as much as 90 per cent.
To enhance the error compensation process, sophisticated procedures have been developed for quickly and efficiently calibrating the machine tool. This is supported by a comprehensive range of industry-leading instrumentation for assessing the errors of the linear and rotary axes.
Additionally, complementary software has been written for volumetric error mapping and machine component capability assessment to provide machine tool users with comprehensive positional and alignment error data for each axis.
The data is stored within the compensation system to enable real-time computation and correction of position errors at the tool point as it moves throughout the working envelope. The information is also supplied in tabulated and graphical formats to assist machine maintenance.
The MTC process During a calibration period, geometric errors are measured on the machine tool using metrology equipment such as laser interferometers, levels and squares. Data files must be in Renishaw ASCII file format and they remain unchanged until recalibration takes place, usually after mechanical work, a collision, an agreed period of time or as a result of statistical process control requirements.
Compensation is applied dynamically throughout the machine volume by calculating the effect of the geometric error sources, ie linear, straightness, angular and squareness errors, throughout the working volume. Tables are created to apply compensation incrementally every time a micron of error is calculated. MTC can be applied to any 3-axis machine and many 5-axis configurations.
The normal procedure for applying compensation on a machine is to measure one of the geometric error components, transfer the data file to the MTC unit, select the file and instruct the software to prepare the tables detailing the compensation. It is then activated in MTC and the error component is measured again to validate the compensation. The sequence is repeated for every error component. Thermal compensation Thermal conditions of a machine tool significantly affect its accuracy, but temperature control of the structure and its operating environment is very expensive.
The Centre for Precision Technologies, again in collaboration with Asquith Butler, has also been working on thermal error compensation techniques to reduce the resulting volumetric errors in normal ambient conditions. It has developed a Thermal Error Compensation (TEC) system that can be integrated into the MTC package. TEC is based on continuously collecting information on the temperature profile of the machine and its environment based on input derived from sensors. This data is continuously fed into the MTC compensation system and used in real time to compute the additional errors at the tool point throughout the working volume due to thermal variances.